Our long-term objective is to understand the role of cortical GABAA receptor-mediated inhibition in the perception of sensory stimuli, and how modulation of GABAA receptors by general anesthetics and changes in cortical inhibition in various neuropathologies alters sensory perception. In this proposal, we seek to understand how GABAergic cells control the timing of action potentials in auditory cortex (ACx). Firing patterns distributed across populations of pyramidal cells in ACx are postulated to represent certain features of acoustic stimuli. Spikes in these population codes are either time-locked to transitions in the stimulus, or comprise temporal patterns generated internally. Evidence suggests that networks of cortical interneurons are involved in establishing both types of patterns. Thalamocortical (TC) afferents activate multiple populations of 'feedforward' GABAergic interneurons in ACx. Lemniscal TC fibers target cells in layers III and IV (LIII/IV), while extralemniscal TC afferents target cells in layer I (LI). Activation of the latter afferents triggers gamma frequency oscillations in ACx. We propose that feedforward intemeurons in ACx regulate spike timing in pyramidal cells and that this capability is enhanced by network interactions among inhibitory cells. We will test the hypothesis that LI interneurons control internally generated firing patterns, while LIII/IV interneurons coordinate firing patterns that are time-locked to the stimulus. Disruption of cortical timing signals has also been postulated to underlie sensory and cognitive impairment during general anesthesia, and modulation of cortical rhythms in ACx is being pursued as a solution to the important medical application of monitoring depth of anesthesia in patients. GABAergic interneurons are central players in normal, pathological and exogenously modulated cortical processing. Thus, understanding how these cells are activated and organized and the functional implications of this structure is critical to understanding cortical information processing. Although evidence abounds that these cells play a pivotal role in setting the response properties of )rincipal cells, we seek to fill the gap in our knowledge of their organization and how they interact with )rincipal cells.